This invention relates to an information recording medium, for example, an optical reflective video disc, a digital disc or a computer memory disc, especially an information recording medium having high recording sensitivity.
As the information recording medium on which information will be recorded and retrieved by a laser beam, methods for using optical characteristics of the medium, for example, a reflection factor (or, reflectivity), a transmission factor (or, transmissivity) or a refractive index, have been proposed. As described in Japanese Laid Open Patent Application Nos. 159,692/57 and 186,243/57, a recording film formed by a double layer structure of an optical absorption layer and a phase change layer has high sensitivity and high recording density. The optical absorption layer has a characteristic of optical absorption to effectively convert light to heat and also works as an optical reflective layer. The phase change layer, that is, a recording layer changes an optical characteristic in response to heat transmitted from the optical absorption layer, in which the laser beam is applied.
In the double layer structure, the reflection factor is determined by an optical interference-effect of the two layers forming the double layer. In order to get a large variation of the optical characteristic, for example, a reflection factor, between pre-recording and post-recording, the minimum value of the reflection factor in pre-recording has been selected. However, the reflection factor varies sharply in response to a thickness of the phase change layer. Namely, a slight deviation of the thickness of the phase change layer varies the total reflection factor of the information recording medium sharply, especially near the minimum value thereof. As a result, a yield rate for producing the information recording medium does not become so high. That is, in the prior art, it has been difficult to get the high sensitive information recording medium with high yield rate.